Surface treated steel sheet with less environmental impact for electronic components, excellent in solder wettability, a rust-proof property and a whisker-proof property

ABSTRACT

The present invention provides a surface treated steel sheet for electronic components which does not include lead, which is a hazardous substance with environmental impact, and, in particular, satisfies the solder wettability after retort treatment, a rust-proof property and a whisker-proof property simultaneously. More specifically, the present invention is a surface treated steel sheet with less environmental impact for electronic components excellent in solder wettability, a rust-proof property and a whisker-proof property and having an Sn—Zn alloy layer which is formed by plating Sn and Zn on a steel sheet or a Ni plated steel sheet and then applying thermal diffusion treatment, or by plating Sn—Zn alloy on a steel sheet or a Ni plated steel sheet, characterized in that the amount of said Sn—Zn alloy layer is not less than 3 g/m 2 , the Zn/Sn ratio (in weight ratio) is 0.01 to 10, more preferably 0.01 to 0.1, and an inorganic film mainly composed of magnesium phosphate is formed in the amount of 1 to 100 mg/m 2  in terms of the amount of P+Mg on said Sn—Zn alloy layer.

TECHNICAL FIELD

The present invention relates to a surface treated steel sheet forelectronic components, used for electronic components of electricproducts, excellent in solderability, a rust-proof property and awhisker-proof property, and not containing substances with environmentalimpact such as lead, etc.

BACKGROUND ART

For electronic components of electric products, a tinplate, aterneplate, a solder plated steel sheet or the like, as a surfacetreated steel sheet particularly excellent in solderability, isgenerally used.

As an example, a surface treated steel sheet having an Sn plated layerof 8.4 to 11.2 g/m² on each surface of a steel sheet (hereunder referredto as “#75 to #100 tinplate”) has been used as a surface treated steelsheet having such excellent solder wettability as to be soldered bydipping the steel sheet in a molten solder bath for a short period oftime. However, a space between electronic components has narrowed due tothe recent downsizing of electric products, and as a result, tinplatehas produced the problems of direct short circuits, the destruction ofan insulating layer, and the like, caused by acicular single crystals(whiskers) grown from a tin plated layer. For that reason, a terneplateor a solder plated steel sheet which does not generate whiskers hasmainly been used.

As methods for preventing the generation of whiskers, advocated have sofar been the method of applying alloy plating (Japanese Examined PatentPublication No. S58-2598, Japanese Unexamined Patent Publication No.S49-129, etc.) and the method of applying post-treatment after plating(Japanese Examined Patent Publication Nos. S56-47955 and S56-47956,Japanese Unexamined Patent Publication Nos. S59-143089 and S62-77481,etc.). However, those methods have scarcely been put to practical usebecause the alloy plating or the post-treatment hinders thesolderability. In the meantime, there is a method which is put topractical use by the optimization of an alloy composition and a chromatetreated layer as disclosed in Japanese Unexamined Patent PublicationNos. H2-270970 and H3-183796.

In recent years, the regulations against hazardous substances withenvironmental impact have been enforced in view of global environmentalproblems and, in particular, hexavalent chromium and lead are objects ofregulation. Therefore, in addition to the need for a material with whichlead-tin solder is substituted, the need for a material as a platedsteel sheet with which a terneplate or a solder plated steel sheet issubstituted is getting increased. In the case of small-sized electroniccomponents in particular, they are soldered by dipping in a moltensolder bath for a short period of time so that they are soldered withhigh efficiency.

Further, as the electronic components are sometimes stored for a longperiod of time, the performance of the electronic components after longstorage is evaluated in the accelerated manner by applying anaccelerating treatment in a retort or the like for experimentallyreproducing the long storage conditions, and a flux is also beingsubstituted with a type of material having a low activity and notcontaining chlorine. For a surface treated steel sheet for electroniccomponents, remarkably excellent solder wettability including solderwettability after the accelerating treatment is required.

As described above, there is a strong demand to provide a surfacetreated steel sheet with less environmental impact for electroniccomponents excellent in both solder wettability and a whisker-proofproperty.

The object of the present invention is to provide a surface treatedsteel sheet for electronic components which does not include lead, whichis a hazardous substance with environmental impact, and, in particular,satisfies the solder wettability after retort treatment, a rust-proofproperty and a whisker-proof property, simultaneously.

DISCLOSURE OF THE INVENTION

The present invention is a surface treated steel sheet capable ofsecuring good a whisker-proof property and a good rust-proof propertywhich have been problems in tinplate, while securing more excellentsolder wettability, after retort treatment, than a terneplate used forapplication to electronic components which are currently soldered bydipping in a molten solder bath for a short period of time.

Such a surface treated steel sheet can be attained by; in a surfacetreated steel sheet for electronic components having an Sn—Zn alloylayer which is formed by plating Sn and Zn on a steel sheet or a Niplated steel sheet and then applying thermal diffusion treatment, or byplating Sn—Zn alloy on a steel sheet or a Ni plated steel sheet:specifying the amount of Sn—Zn alloy and a Zn/Sn ratio; and applyingthereon an inorganic film mainly composed of magnesium phosphate as thesubstitution of a conventional chromate film.

That is, the present invention is a surface treated steel sheet withless environmental impact for electronic components: excellent in solderwettability, a rust-proof property and a whisker-proof property; andhaving an Sn—Zn alloy layer which is formed by plating Sn and Zn on asteel sheet or a Ni plated steel sheet and then applying thermaldiffusion treatment, or by plating Sn—Zn alloy, characterized in that:the amount of said Sn—Zn alloy layer is not less than 3 g/m²; the Zn/Snratio (in weight ratio) is 0.01 to 10, more preferably 0.01 to 0.1; andan inorganic film mainly composed of magnesium phosphate is formed inthe amount of 1 to 100 mg/m² in terms of the amount of P+Mg on saidSn—Zn alloy layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing the surface layer structure in thecross section of the plated layer of a surface treated steel sheetaccording to the present invention.

FIG. 2 is a map showing the relation among the P+Mg deposited amount ofan inorganic film, the Zn/Sn ratio of an Sn—Zn alloy layer and solderwettability.

FIG. 3 is a graph showing the relation between the Zn/Sn ratio of anSn—Zn alloy layer and a whisker-proof property.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will hereunder be explained in detail. Firstly,the reasons for limiting the scope of a surface treated steel sheetaccording to the present invention will be explained.

FIG. 1 schematically shows the surface layer structure in the crosssection of the plated layer of a surface treated steel sheet accordingto the present invention. In the figure: numeral 1 designates an Sn—Znalloy layer which is formed by plating Sn and Zn on a steel sheet or aNi plated steel sheet (not shown in the figure) and then applyingthermal diffusion treatment, or by plating Sn—Zn alloy on a steel sheetor a Ni plated steel sheet; and numeral 2 an inorganic film mainlycomposed of magnesium phosphate and formed on an Sn—Zn alloy layer 1,which is most intrinsic to the present invention.

An Sn—Zn alloy layer is a plated film which constitutes the basis of thepresent invention, and the deposited amount is required to be at least3.0 g/m² or more for securing good solder wettability and a goodrust-proof property. The upper limit thereof is not particularlydetermined in the present invention, but it is preferable that the upperlimit is about 50 g/m² from the viewpoint of the cost.

With regard to methods for forming an Sn—Zn alloy layer, in addition tothe method of forming the alloy layer by electroplating Sn and Zn on asteel sheet or a Ni plated steel sheet and then applying thermaldiffusion treatment to the steel sheet or the Ni plated steel sheet,there are a method of directly coating a steel sheet with Sn—Zn alloyusing electroplating and a method of dipping a steel sheet in a moltenSn—Zn alloy bath, namely the hot dipping method. Further, by using an Niplated substrate steel sheet instead of said steel sheet in the abovethree methods as disclosed in Japanese Unexamined Patent PublicationNos. H2-270970 and H3-183796, it is possible to form an Ni plated layeror an Fe—Ni diffusion layer at each interface on a steel sheet and anSn—Zn alloy layer on the surface layer. Note that in the presentinvention an Sn—Zn alloy layer is formed on both surfaces of a steelsheet or a Ni plated steel sheet. Accordingly, the present inventiondoes not particularly restrict a method for forming an Sn—Zn alloylayer.

By forming an Ni plated substrate layer, when an Sn—Zn alloy layer isthin, the Sn—Zn alloy layer is uniformed and the rust-proof propertyimproves.

Next, the restrictions related to the Zn/Sn ratio (in weight ratio) ofan Sn—Zn alloy layer and an antioxidation film will be explained. Byforming a thin inorganic film, which is mainly composed of magnesiumphosphate in the amount of 1 to 100 mg/m² in terms of the amount ofP+Mg, on an Sn—Zn alloy layer having a deposited amount of not less than3 g/m² and a Zn/Sn ratio (in weight ratio) of not more than 10.0 bydipping in a bath mainly composed of magnesium biphosphate solution andthen drying, it becomes possible to suppress the growth of the oxidefilm after accelerating treatment such as retort treatment and to secureexcellent solder wettability and a rust-proof property of the same levelas that of a conventional chromate film, as seen in FIG. 2.

Further, by setting the lower limit of a Zn/Sn ratio (in weight ratio)at not less than 0.01, it becomes possible to secure a goodwhisker-proof property as shown in FIG. 3.

FIG. 2 shows the relationships between the deposited amount of P+Mgwhich represents the amount of an inorganic film mainly composed ofmagnesium phosphate and formed on a surface layer, the Zn/Sn ratio (inweight ratio) of an Sn—Zn alloy layer, and solder wettability, when anSn—Zn alloy amount is 5.0 to 20.0 g/m². In this case, the solderwettability was evaluated by using a device to record the deteriorationof a solder meniscus with age, employing Sn—Ag lead-free solder as thesolder and two kinds of flux, inactive type and active type, as theflux, and measuring the wettability after test pieces were subjected tothe accelerating treatment in a retort at 105° C. for 8 hours.

The results of the evaluation are expressed by the mark ⊚ when the wettime (zero cross time) is within 3 seconds under the inactive flux, ◯when the same is within 3 seconds under the active flux, Δ when the sameis 3 to 5 seconds under the active flux and X when the same is not lessthan 5 seconds under the active flux.

As is shown in FIG. 2, the range of the Zn/Sn ratio where the solderwettability is good is not more than 10.0 and, in particular, very goodwettability is obtained in the range not more than 0.1 even under theinactive flux. With regard to the P+Mg deposited amount, in the casethat the deposited amount is zero, the evaluation result is Δ when theZn—Sn ratio (in weight ratio) is in the range of 0.01 to 0.1 and X whenthe same is not less than 1, and therefore, in order for the inorganicfilm to function as an antioxidation film, the P+Mg deposited amountmust be not less than 1 mg/m². With regard to the upper limit of theP+Mg deposited amount, though it varies depending on the type of acoating method, there is a tendency to hinder the solder wettabilitywhen the deposited amount exceeds 100 mg/m². Therefore, it is preferablethat the P+Mg deposited amount is not more than 100 mg/m².

FIG. 3 shows the result on the relation between the Zn/Sn ratio (inweight ratio) of an Sn—Zn alloy layer and the whisker-proof property.The test for evaluating the whisker-proof property was carried out bysubjecting the test pieces to 90 degree bending and bulging processing,and then aging for 3 months in the atmosphere of 60° C. in temperatureand 90% in relative humidity which was similar to the atmosphereemployed in the moisture resistance test. The evaluation itself wascarried out by visual inspection and scanning electron microscope, andthe results of the evaluation are expressed by the mark ◯ when theoccurrence of whiskers is less than 50 μm and X when the same is notless than 50 μm.

As is shown in FIG. 3, with regard to the whisker-proof property, theoccurrence of whiskers is less than 50 μm when the Zn/Sn ratio (inweight ratio) is not less than 0.01. From the above explanation, therange of the Zn/Sn ratio (in weight ratio) is limited to not less than0.01 from the viewpoint of the whisker-proof property, not more than10.0 from the viewpoint of the solder wettability under the active flux,and not more than 0.1 from the viewpoint of the solder wettability underthe inactive flux.

EXAMPLE

The present invention will further be explained based on the exampleshereunder. The evaluation results of the performances in the exampleswherein detailed conditions are varied and the comparative examples aresummarized in Table 1.

Example 1

Low carbon cold rolled steel sheets produced by cold rolling andannealing in a usual manner were subjected to degreasing and pickling,by a usual method, and then coated, in order, with Ni plating under thetreatment conditions shown in the item (1), Sn plating under thetreatment conditions shown in the item (2) and Zn plating under thetreatment conditions shown in the item (3). In succession, the steelsheets were subjected to a heat treatment at a steel sheet surfacetemperature of 250 to 350° C. for not less than 0.5 second in theatmosphere using an electric resistance heating method, and the platedfilms mainly composed of Sn—Zn binary alloy were formed thereon.Further, the steel sheets were subjected to post-treatment under theconditions shown in the items (4) and (5) and, after that, to variouskinds of evaluation tests.

(1) Ni plating {circle around (1)} Bath condition NiSO₄.7H₂O: 200-300g/L (liter) H₂SO₄: 0-50 g/L (liter) H₃BO₃: 40 g/L (liter) {circle around(2)} Plating condition Bath temperature: 40-50° C. Current density: 5-30A/dm² (2) Sn plating {circle around (1)} Bath condition Tin sulfate:20-30 g/L (liter) Phenolsulfonic acid: 20-30 g/L (liter) Ethoxylationα-naphtholsulfonic acid: 2-3 g/L (liter) {circle around (2)} Platingcondition Bath temperature: 35-45° C. Current density: 2-30 A/dm² (3) Znplating {circle around (1)} Bath condition ZnSO₄.7H₂O: 200-400 g/L(liter) Na₂SO₄: 50-150 g/L (liter) {circle around (2)} Plating conditionBath temperature: 40-50° C. Current density: 5-30 A/dm2 (4) Treatmentfor removing surface layer oxide film Dipping time: 3 seconds {circlearound (1)} Bath condition H₂SO₄: 10-20 g/L (liter) {circle around (2)}Bath temperature Ordinary temperature (20-30° C.) (5) Antioxidation filmtreatment {circle around (1)} Bath condition magnesium biphosphatesolution: 1-20 g/L (liter) {circle around (2)} Treatment Bath: Ordinarytemperature condition to 50° C. (dip for 3-5 seconds)

Example 2

Low carbon cold rolled steel sheets produced by cold rolling andannealing, in a usual manner, were subjected to degreasing and picklingby a usual method, and then, in order, coated with Ni plating under thetreatment conditions shown in the item (1) of Example 1, and Sn—Zn alloyplating under the treatment conditions shown in the item (6) below andsubjected to the treatment for removing surface layer oxide films underthe treatment conditions shown in the item (4) of Example 1. Insuccession, the steel sheets were subjected to an antioxidation filmtreatment under the conditions shown in the item (5) of Example 1 and,after that, to various kinds of evaluation tests.

(6) Sn—Zn alloy hot dip plating {circle around (1)} Bath condition Sn—Znalloy {circle around (2)} Plating conditions Bath temperature: 250-300°C. Dipping time: 1 second Plating amount: 30-40 g/m² (wiping control)

Example 3

Low carbon cold rolled steel sheets produced by cold rolling andannealing in a usual manner were subjected to degreasing and pickling bya usual method, and then, in order, coated with Ni plating under thetreatment conditions shown in the item (1) of Example 1, and Sn—Zn alloyplating under the treatment conditions shown in the item (7) below andsubjected to the treatment for removing surface layer oxide films underthe treatment conditions shown in the item (4) of Example 1. Insuccession, the steel sheets were subjected to an antioxidation filmtreatment under the conditions shown in the item (5) of Example 1 and,after that, to various kinds of evaluation tests.

(7) Sn—Zn alloy electroplating {circle around (1)} Bath conditionsAlkanolsulfonic acid: 10-200 g/L (liter) Bivalent zinc: 1-50 g/L (liter)Bivalent Tin: 100-500 g/L (liter) {circle around (2)} Plating conditionsBath temperature: 50-60° C. Current density: 10-200 A/dm²

Comparative Example 1—1

Comparative Example 1—1 was prepared by applying a chromate treatmentunder the conditions shown in the item (8) below instead of thetreatments shown in the items (4) and (5) of Example 1, with the otherconditions being the same as Example 1.

(8) Chromate treatment {circle around (1)} Bath condition CrO₃: 50-100g/L (liter) {circle around (2)} Bath temperature: 40-50° C. (dip for 5seconds)

Comparative Example 1-2

Comparative Example 1-2 was prepared by eliminating the chromatetreatment shown in the item (8) from Comparative Example 1—1, with theother conditions being the same as Example 1.

Comparative Example 2

Comparative Example 2 is an electroplated tinplate having the Sn platingamount of 11.2 g/m² per each surface (referred to as “#100 tinplate”),and the plate is subjected to various kinds of evaluation tests.

Comparative Example 3

Comparative Example 3 is a lead plated steel sheet (referred to as“terneplate”) having the Pb plating amount of 30 g/m² per each surface,and the plate is subjected to various kinds of evaluation tests.

The above-mentioned Examples according to the present invention andComparative Examples were subjected to the evaluation tests shown in theitems (a) to (c) below and their properties were evaluated. It should benoted that, with regard to Examples, Sn—Zn alloy plating amount (g/m²),Zn/Sn ratio (in weight ratio) and the P+Mg deposited amount weremeasured by the methods shown in the items (1) to (3) below before theywere subjected to the evaluation.

(a) Solder Wettability Test

The solder wettability test was carried out by using a device to recordthe deterioration of a solder meniscus with age (SWET-2100, manufacturedby Tarutin Kester, Co., Ltd.), employing Sn—Ag—Bi lead-free solder(SA2515, manufactured by Tarutin Kester, Co., Ltd.) as the solder andtwo kinds of flux, non-chloride flux (NA200, manufactured by TamuraGiken Co., Ltd.) and active flux containing chlorine (NS828,manufactured by Nihon Superior Co., Ltd.) as the flux, and wettabilitywas measured after the test pieces were subjected to the acceleratingtreatment in a retort at 105° C. for 8 hours.

The results of the evaluation are expressed by the mark ⊚ when the wettime (zero cross time) is within 3 seconds under the inactive flux, ◯when the same is within 3 seconds under the active flux, Δ when the sameis 3 to 5 seconds under the active flux and X when the same is not lessthan 5 seconds under the active flux.

(b) Whisker-Proof Test

The whisker-proof test was carried out by subjecting the test pieces to90 degree bending and bulging processing, and then aging for 3 months inthe atmosphere of 60° C. in temperature and 90% in relative humiditywhich was similar to the atmosphere employed in the moisture resistancetest. The evaluation itself was carried out by visual inspection and bya scanning electron microscope, and the results of the evaluation areexpressed by the mark ◯ when the occurrence of whiskers is less than 50μm and X when the same is not less than 50 μm.

(c) Rust-Proof Test

The rust-proof test was carried out by subjecting the test pieces to the72 hour continuous salt spraying test specified in JIS (JapaneseIndustrial Standards) Z 2371, and measuring the area ratio of generatedred rust in terms of percentage.

(1) Measurement of Sn—Zn Alloy Amount (g/m²)

Each weight of Sn and Zn was determined by applying each value of Sn andZn measured by a fluorescent X-ray spectrometer to the calibration curveof each weight of Sn and Zn prepared beforehand, and then the Sn—Znalloy amount was obtained by summing each weight.

(2) Measurement of Zn/Sn Ratio (In Weight Ratio)

The Zn/Sn ratio was calculated from each weight of Sn and Zn obtained bythe same method as shown in the item (1).

(3) Measurement of P+Mg Deposited Amount

The weight of P was determined by applying the value of P measured by afluorescent X-ray spectrometer to the calibration curve of the weight ofP prepared beforehand, the weight of Mg was determined by applying thevalue of Mg measured in a solution wherein a coating film is dissolvedby acid using a high frequency inductively coupled plasma emissionspectrometer to the calibration curve of the weight of Mg preparedbeforehand, and then the P+Mg deposited amount was obtained by summingeach weight of P and Mg.

Table 1 shows the details and the evaluation results of the performancesof Examples and Comparative Examples in a summarized manner.

The evaluation results of the performances of Examples are shown inExamples 1—1 to 1-4 which an Sn—Zn alloy layer is formed by applying thethermal diffusion alloying treatment after electroplating of Sn and Zn,in Examples 2-1 and 2—2 which an Sn—Zn alloy layer is formed by applyinghot dip plating and in Example 3 which an Sn—Zn alloy layer is formed byapplying alloy electroplating.

The evaluation results of the performances of Comparative Examples areshown in Comparative Example 1—1 in the case of applying the thermaldiffusion treatment after electroplating and then forming a chromatefilm, in Comparative Example 1-2 in the case of not applying chromatetreatment and in Comparative Examples 2 and 3 in the cases of #100tinplate and terneplate, respectively.

As seen in Examples, the case of employing the inorganic film containingP+Mg is better than the case of employing the chromate treatment insolder wettability after the accelerating treatment in a retort, shows asimilar good performance in the rust-proof property, and shows moreexcellent performance than the #100 tinplate and the terneplate whichare included in Comparative Examples.

TABLE 1 Plating layer Surface film Sn-Zn P + Mg (a) (b) (c) alloy Zn/Snratio deposited Solder Whisker- Incidence of Plating amount (in weightTreatment amount wett- Proof red rust method (g/m³) ratio) Condition(mg/m²) ability property (area percent) Example 1-1 Electroplating + 100.1 (5) 10 ⊚ ◯ 2 thermal diffusion treatment Example 1-2Electroplating + 10 10 (5) 25 ◯ ◯ 1 thermal diffusion treatment Example1-3 Electroplating + 15 0.01 (5) 14 ⊚ ◯ 1 thermal diffusion treatmentExample 1-4 Electroplating + 5 0.1 (5) 17 ⊚ ◯ 3 thermal diffusiontreatment Example 2-1 Electroplating + 35 1 (5) 20 ◯ ◯ 1 hot dip platingExample 2-2 Electroplating + 35 0.1 (5) 11 ⊚ ◯ 2 hot dip plating Example3 Alloy 10 0.1 (5) 14 ⊚ ◯ 1 electroplating Comparative Electroplating +10 0.05 (8) 2 mg/m² x ◯ 3 example 1-1 thermal diffusion metallictreatment chromium conversion Comparative Electroplating + 10 0.05 None— Δ ◯ 15 example 1-2 thermal diffusion treatment ComparativeElectroplating + 12 0 Dichromic 5 mg/m² Δ x 20 example 2 thermaldiffusion acid, in metallic treatment electro- chromium (#100 tinplate)lysis conversion Comparative Hot dip plating 35 — — None Δ ◯ 1 example 3(terneplate)

INDUSTRIAL APPLICABILITY

As explained above, a surface treated steel sheet according to thepresent invention has excellent performances in solder wettability aftera retort treatment and a rust-proof property and a whisker-proofproperty which are suitable for electronic components. The presentinvention makes it possible to supply a surface treated steel sheet withless environmental impact for electronic components.

What is claimed is:
 1. A surface treated steel sheet with lessenvironmental impact for electronic components excellent in solderwettability, a rust-proof property and a whisker-proof property andhaving an Sn—Zn alloy layer which is formed by plating Sn and Zn on asteel sheet or a Ni plated steel sheet and then applying thermaldiffusion treatment, or by plating Sn—Zn alloy on a steel sheet or a Niplated steel sheet, characterized in that the amount of said Sn—Zn alloylayer is not less than 3 g/m², the Zn/Sn ratio (in weight ratio) is 0.01to 10 and an inorganic film mainly composed of magnesium phosphate isformed in the amount of 1 to 100 mg/m² in terms of the amount of P+Mg onsaid Sn—Zn alloy layer.
 2. A surface treated steel sheet with lessenvironmental impact for electronic components excellent in solderwettability, a rust-proof property and a whisker-proof propertyaccording to claim 1, characterized in that the Zn/Sn ratio (in weightratio) of an Sn—Zn alloy layer is 0.01 to 0.1.